Imaging elements, such as those described in U.S. Pat. Nos. 4,661,429 and 4,650,734, as well as U.S. Pat. No. 3,681,066, German Democratic Republic Patent No. 155,270 and Japanese Patent No. 103,260, are useful for "expose once, print many times" applications, also known as electrographic printing, or xeroprinting. These elements differ from others in that they comprise a conductive layer in electrical contact with an acid photogenerating layer which (i) is free from polymerizable monomers and (ii) comprises an electrically insulating binder and an acid photogenerator. Other important differences between these elements and similar prior art systems are that the light-induced conductivity is persistent for much longer periods of time and that it is bipolar, i.e., the element can be charged to either a positive or negative potential with equivalent results. Furthermore, unlike other photoelectrographic elements, the elements of the type described in U.S. Pat. No. 4,661,429 require no pre- or post-exposure treatment or processing of any kind prior to their use in an electrographic printing process, nor does the electrographic printing cycle, as further described hereinafter, require any further exposure steps.
One potential drawback to elements of this type is that the electroconductive latent image may be invisible or poorly visible, especially when the actinic radiation is not in the visible region of the electromagnetic spectrum, e.g., ultraviolet, infrared, or x-ray radiation. In many applications employing these and similar elements, it would be desirable to visualize the electroconductive latent image.
Dyes have been used in conjunction with acid photogenerators to effect color density or hue shifts for a variety of applications outside of elements for photoelectrographic printing processes. U.S. Pat. No. 4,701,402 to Patel et al. utilizes an element containing a bleachable dye in reactive association with an iodonium ion for making overhead transparencies or color slides by exposing a printed or typed original. European Patent Application No. 175,238A to Ishii et al., discloses a similar non-electrographic, direct imaging process with a photosensitive composition including a photoactivator, such as an aromatic halonium salt. U.S. Pat. No. 4,659,649 to Dickinson et al. discloses a coating for silk screen stencils or lithographic printing plates containing a photocurable resin, an onium salt, and a dyestuff. European Patent Application No. 290,750A to Mullis discloses a composition containing a photoacid-progenitor and a dye which can be incorporated in various materials to indicate exposure to U.V. radiation. D. R. McKean et al., Proceedings of the ACS Division of Polymeric Materials: Science & Engineering, 60, 45 (1989) incorporates merocyanine dyes into a photoresist formulation containing a triphenylsulfonium salt to achieve bleaching proportional to U.V. irradiance and to measure yields for photoacid Production. U.S. Pat. Nos. 4,672,021 and 4,777,111 to Blumel et al. relate to a photographic element containing a light sensitive material which generates an acid upon exposure and a dye.
Dyes have also been used in conjunction with radical photogenerators to effect color density or hue shifts for a variety applications outside of elements for photoelectrographic printing processes. For example, imaging elements comprising one or more layers over a conductive substrate and which further comprise reactive combinations of polyhalogen compounds or diazonium salts and dye precursor compounds have been disclosed in U.S. Pat. No. 3,765,883 to Endo et al., U.S. Pat. No. 4,042,388 to Inoue et al., British Patent No. 1,289,529 to Canon KK, British Patent No. 1,424,323 to Vanheertum and Japanese Patent Application Nos. 74/029,466 and 53/003,827. Japanese Patent Application No. 56/025,744 to Ricoh describes similar elements in which a color forming layer is used as an integral mask in conjunction with photoconductive layers. Although the elements of these references are useful for certain imaging applications, they do not result in an electroconductive latent image.
There have also been disclosures relating to various photoelectrographic imaging elements, comprising a conductive layer and a photosensitive layer with an acid photogenerator and a dye. U.S. Pat. No. 3,879,197 to Bartlett et al. utilizes a photosensitive layer containing leuco xanthene dyes and organic halogen compounds capable of forming acids with no mention of a color density or hue shift. U.S. Pat. No. 4,945,020 to Kempf et al. relates to an element with a photosensitive layer comprising a leuco dye and a nonionic halogenated compound. This element has a slow charge decay rate which would preclude its use in high speed printing processes. Such elements are further limited to a print-out process described infra.